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Jean-Sylvain Bailly, (born Sept. 15, 1736, Paris—died Nov. 12, 1793, Paris), French astronomer noted for his computation of an orbit for Halley’s Comet (1759) and for his studies of the four satellites of Jupiter then known. He was also a statesman who took part in the revolutionary events of his age.

Bailly began his study of Halley’s Comet in 1759. One year later he established an observatory where he could undertake observations of Jupiter’s satellites. He was elected to the Académie des Sciences in 1763. His major works include Essai sur la théorie des satellites de Jupiter (1766; “Essay on the Theory of Jupiter’s Satellites”) and Mémoires sur les inégalités de la lumière des satellites de Jupiter (1771; “Memoirs on the Uneven Illumination of Jupiter’s Satellites”).

Later in his career Bailly wrote widely read histories of astronomy: Histoire de l’astronomie ancienne depuis son origine jusqu’à l’établissement de l’école d’Alexandrie (1775; “History of Ancient Astronomy from Its Origin to the Establishment of the School of Alexandria”) and Histoire de l’astronomie moderne depuis la fondation de l’école d’Alexandrie, jusqu’à l’époque de M.D.CC.XXX (1779; “History of Modern Astronomy from the Foundation of the School of Alexandria to the Epoch 1730”). However, these were soon superceded by the more scholarly histories of French astronomer Jean-Baptiste-Joseph Delambre. Bailly’s Traité de l’astronomie indienne et orientale (1787; “Treatise on Indian and Oriental Astronomy”) was part of a wave of 18th-century interest in the history and methods of non-Western astronomy. In 1784 Bailly was a member of a royal commission appointed to investigate the claims of animal magnetism made by German physician Franz Mesmer and was the author of the report concluding that Mesmer’s magnetic fluid probably did not exist and that the effects on his human subjects were likely due to imagination and imitation.

The French Revolution interrupted his studies. Elected deputy from Paris to the Estates-General, he was chosen president of the third estate on May 5, 1789, and led the famous proceedings in the Tennis Court on June 20. He was proclaimed the first mayor of Paris on July 15, 1789. He was reelected mayor in August 1790 but lost popularity, particularly after his order to the national guard to disperse a riotous crowd led to the massacre of the Champ de Mars on July 17, 1791. Bailly retired on Nov. 16, 1791, and went to Nantes in July 1792, where he composed Mémoires d’un témoin de la Révolution (“Memoirs of a Witness of the Revolution”), an incomplete narrative of the extraordinary events of his public life. Late in 1793 Bailly went to Melun to join his friend, scientist Pierre-Simon Laplace, but was recognized, arrested, and taken before the revolutionary tribunal at Paris on November 10. He was subsequently guillotined.

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